No problem. moved.
Have you specify the source IP when your configure each node?
Looks like lslb has problem connecting to all backend servers. could it be a problem with NIC port, switch port? If you use dedicate connection communicate with backend servers, you can check the packet loss of that specific NIC.
LSLB uses persistent connections, while nginx does not, there could be more ESTABLISHED connections with LSLB. Is there a firewall between LSLB and web servers?

If you do think it is a LSLB bug, could you strace lslbd while the problem is happening to help analyze the cause of the problem?

Since it get worse when server is busy, my guess is, after the Gb port upgrade for the DB server, the burst bandwidth usage is higher and causes some packet being dropped.
Maybe there are some kind of bandwidth cap on the switch port if server are interconnected via a VLAN instead of a dedicate switch.

Please download 1.7 release and update again, I am not sure it is something wrong with interpreting getsockopt() results or not.
Also strace need to be updated to the latest release to get correct result value for getsockopt() on 64bit Linux.